The invention relates to a radiation detector for an automatic x-ray exposure timer composed of two walls made of synthetic material fixed at a distance from one another, and each of which is covered with a shielding layer on its exterior side and with an electrode layer on the side facing the other wall.
It is an obvious procedure to arrange a radiation detector of this type in front of an x-ray film, viewed in the direction of radiation. In conjunction with an electric circuit arrangement, this radiation detector forms a signal which is proportional to the radiation dose. The x-ray tube is switched off when this signal and thus the radiation dose reaches a specific value.
In order to produce x-ray photographs, not only x-ray films are used, but in the field known as xeroradiography, altogether conventional paper sheets are also employed. Xeroradiography has been proven successful particularly in the production of mammograms, or photographs of extremeties. A photographic cassette with an electro-photographic exposure layer is here introduced in a support mounting behind the photographic subject. Upon irradiation with x-rays, the exposure layer becomes electrically conductive, thereby serving the production of x-ray images. An arrangement for the electro-photographic exposure of radiographs is known, for example, from the article "Xeroradiography" by J. W. Boag in Phys. Med. Biol. volume 18 (1973), number 1, pages 5 through 37, particularly page 25.
In order to automatically control the exposure of a xeroradiographic device, the known radiation detectors can be arranged only behind the photographic cassette, since in xerographic image production, the contours of the edges are over-accentuated, and the known radiation detectors would produce shadows on the photographic exposures. However, the arrangement of a radiation detector behind the photographic cassette is not a satisfactory solution in xeroradiography, since the transmitted radiation transmitted by such a cassette and still available for measurement by the detector is very low. A xeroradiographic cassette, namely, absorbs more than 80% of the radiation, and in unfavorable instances, up to 99%. As a consequence, the cassette-transmitted dose available for sensing and control of the automatic exposure timer is to a considerable extent dependent upon the electric potential utilized in the particular exposure and the thickness of the particular subject. These difficulties can be avoided if the dose impinging upon the cassette is measured by a radiation detector arranged in front of the cassette.